This invention is directed to a polypropylene microporous membrane, made from a beta-nucleated precursor, for use as a battery separator.
Polypropylene microporous membranes, made from beta-nucleated precursors, are known. U.S. Pat. Nos. 5,134,174; 5,231,126; 5,317,035; and 5,594,070; EPO Publication No. 632,095; Japanese Kokai No. 7-118429; Chu, F. et al., xe2x80x9cMicrovoid formation process during the plastic deformation of Beta-form polypropylenexe2x80x9d, POLYMER v34 n16, 1994; Chu, F. et al., xe2x80x9cCrystal transformation and micropore formation during uniaxial drawing of Beta-form polypropylene filmxe2x80x9d, POLYMER v36 n13, 1995; Ikeda, N. et al., xe2x80x9cNJ-Star NU-100: A Novel Beta-Nucleator for Polypropylenexe2x80x9d, Polypropylene and World Congress, Sep. 18-20, 1996; Zhu, W. et al., xe2x80x9cA New Polypropylene Microporous Filmxe2x80x9d, Polymers for Advanced Technologies, v7, 1996. Such membranes have been suggested for use as battery separators. U.S. Pat. No. 5,134,174; EPO Publication No. 632,095; Japanese Kokai No. 7-118429. Beta-nucleating agents for polypropylene are also known. U.S. Pat. Nos. 5,134,174; 5,231,126; 5,317,035; and 5,594,070; EPO Publication Nos. 557,721 and 632,095; Japanese Kokai Nos. 7-118429 and 9-176352; Chu, F. et al., xe2x80x9cMicrovoid formation process during the plastic deformation of Beta-form polypropylenexe2x80x9d, POLYMER v34 n16, 1994; Chu, F. et al., xe2x80x9cCrystal transformation and micropore formation during uniaxial drawing of Beta-form polypropylene filmxe2x80x9d, POLYMER v36 n13, 1995; Ikeda, N. et al., xe2x80x9cNJ-Star NU-100: A Novel Beta-Nucleator for Polypropylenexe2x80x9d, Polypropylene and World Congress, Sep. 18-20, 1996; Zhu, W. et al., xe2x80x9cA New Polypropylene Microporous Filmxe2x80x9d, Polymers for Advanced Technologies, v7, 1996.
Commercially viable battery separators need to have a balance of properties. Two of these properties are electrical resistance (ER) and strength, typically measured as puncture strength (PS). Electrical resistance is a measure of the resistance to electrical flow between the anode and cathode and across the separator, and is generally preferred to be as low as possible. The micropores of the battery separator form channels through which electrolyte is in contact with the anode and cathode. Puncture strength is for withstanding the rigors of battery manufacture, among other things. Battery separators are usually extremely thin (approximately 1 mil or 25 micron), and are sandwiched between the anode and cathode which have sufficient surface roughness to cause puncture during the winding or folding of the sandwich. Puncture of the separator may lead to direct contact between the anode and cathode, which renders the battery unsuitable for use.
In U.S. Pat. No. 5,134,174, EPO Publication No. 632,095, and Japanese Kokai No. 7-118429, polypropylene microporous films, made from beta-nucleated precusors, for use as battery separator are disclosed. These films, while theoretically functional as separators, are limited. For example, the limitation of the films disclosed in the U.S. and the Japanese references arises from poor puncture strength. The puncture strength is apparent from the stretching (or drawing) conditions, as well as, the pore size, and the porosity.
Accordingly, there is a need for a polypropylene microporous membrane, made from a beta-nucleated precursor, that is commercially viable as a battery separator.
A battery separator is a polypropylene microporous membrane, made from a beta-nucleated precursor, and having an electrical resistance of less than 30 ohms-inches per mil, and a puncture strength of greater than 400 grams-force per mil.
A battery, or an electrochemical cell, is a device generally comprising an anode, a cathode, an electrolyte, and a separator. Batteries may be used in portable devices, such as computers, cellular telephones, or the like, or in electric vehicles.
Battery separator, as used herein, refers to a microporous membrane that is used to separate the anode and the cathode, thereby preventing their direct contact, and to contain, in the micropores, the electrolyte. The separator may be a monolayer or a multilayer structure (i.e., a sandwich in which the individual layers may be the same or different) in which the disclosed film may be one of those layers. Other layers may enhance safety (i.e., low-melting or shutdown function, e.g., U.S. Pat. No. 5,691,077) or enhance strength (i.e., cross-plied, e.g., U.S. Pat. No. 5,667,911). The battery separator has a thickness less than 3 mils and preferable less than 1.5 mils.
The battery separator disclosed herein is a polypropylene microporous membrane made from a beta-nucleated precursor. Polypropylene refers to any polymer (e.g., homo -or co-polymer) of predominantly propylene monomers. Preferably, the polypropylene is an isotactic, homopolymer with a melt flow index (MFI) of less than ( less than ) 10. More preferably, the MFI is less than 5. Exemplary polypropylenes include: Huntsman Chemical Corp. of Woodbury, N.J. product 5550 (MFI-5.5); Exxon Chemical Co. of Houston, Tex. product Escorene PP 4352 FI (MFI-about 3) and PP 4292 (MFI-1.5); Aristech Chemical Corp. of Pittsburgh, Pa. product BEPOL (MFI-0.7).
Beta-nucleated precursor refers to a pre-stretched polypropylene film having a beta-crystal structure. Beta-crystals are meta-stable and will revert to alpha-crystals when subjected to a combination of heat and stress. Beta-crystal may be formed in the polypropylene by any number of known techniques, but, the use of a beta-nucleating agent (or beta-nucleator) is preferred. See: U.S. Pat. Nos. 5,134,174; 5,231,126; 5,317,035; and 5,594,070; EPO Publication No. 632,095; Japanese Kokai Nos. 7-118429 and 9-176352; Chu, F. et al., xe2x80x9cMicrovoid formation process during the plastic deformation of Beta-form polypropylenexe2x80x9d, POLYMER v34 n16, 1994; Chu, F. et al., xe2x80x9cCrystal transformation and micropore formation during uniaxial drawing of Beta-form polypropylene filmxe2x80x9d, POLYMER v36 n13, 1995; Ikeda, N. et al., xe2x80x9cNJ-Star NU-100: A Novel Beta-Nucleator for Polypropylenexe2x80x9d, Polypropylene and World Congress, Sep. 18-20, 1996; Zhu, W. et al., xe2x80x9cA New Polypropylene Microporous Filmxe2x80x9d, Polymers for Advanced Technologies, v7, 1996, each of which is incorporated herein by reference. Beta-nucleating agents are commercially available, for example NJ-STAR NU-100 is available from New Japan Chemical Co., Ltd., Osaka, Japan. The amount of beta-crystals in the precursor should be on the order of 45 to 70% as measured by a differential scanning calorimeter (DSC) technique (sample size-10 milligrams, heating rate-10xc2x0/min, heating range-25xc2x0 C. to 200xc2x0 C., using Seiko Instrument Inc.""s model 220 C). By this technique, the amount of beta-crystal is reported as the ratio of beta-crystal (measured as the area under the beta-crystal peak of the DSC trace) to the sum of beta- and alpha-crystal (the sum of the areas under the beta-crystal and the alpha-crystal peaks). The beta-crystal content of the precursor is not preferably maximized.
The porosity of the foregoing membranes should be in the range of 40-65%, preferably 45-60%, and more preferably 47-57%. Porosity (%) is: [1-(apparent density of membrane/resin density)]xc3x97100.
The average pore size should be on the order of 0.03 to 0.25 microns as measured from scanning electron microscope (SEM) photograph, magnification 20,000xc3x97. A preferred average pore size is in the range of 0.04-0.10 with the distribution skewed toward the low end of the range.
The electrical resistance of the separator should be less than 30 ohms-inches per mil of thickness. A more preferred range for electrical resistance is less than 20 ohm-inches per mil. The electrical resistance (or resistivity) is measured as follows: A R.A.I. AC Milliohm Resistance Meter, Model 2401 and R.A.I. test cell electrode (from RAI Research Corp. Hauppauge, N.Y.) are used. A 31% by wt KOH solution is used to wet the sample (samples should be methanol primed to ensure complete wetout then soaked in solution for 8 to 24 hours before testing). Samples should not be dry when tested. Three samples of material are tested and averaged. The results, reported in milliohm-inch2, are then divided by the material thickness and reported as ohm-inches per mil.
A puncture strength of greater than 400 grams force per mil of thickness is preferred. There is no upper range on the puncture strength as in commercial operation, the greater the strength, the more preferred the separator is. The test procedure is as follows: A Mitech Stevens LFRA Texture Analyzer with a needle (1.65 mm in diameter, 0.5 mm radius tip) is used. The rate of descent is 2 mm/sec and the maximum amount of deflection is 6 mm. The film is held taut in a clamping device with a central opening of 11.3 mm in diameter. Ten measurements are taken, averaged and normalized to one mil of thickness.
The separator is preferably manufactured by the following xe2x80x98dry-stretchxe2x80x99 or OPP (oriented polypropylene) technique: the polypropylene resin is doped with the nucleator; the resin is extruded; a precursor is formed; and the precursor is stretched (drawn) into the battery separator. To obtain the balance of physical properties (e.g., porosity, average pore size, electrical resistance, and puncture strength), the precursor preferably has a beta-crystal content of 45-70%, more preferably 46-60%, and preferably undergoes a total stretch (TS) ranging from at least 16xc2x17 to 36xc2x17, preferably at least 16xc2x14 to 36xc2x17, and most preferably 16xc2x14 to 20xc2x14.
The beta-nucleator may be added to the resin during resin polymerization, by compounding, or at the extruder. Sufficient nucleator should be added to insure that the precursor""s required content of beta-crystal is obtained prior to stretching.
After extrusion of the precursor, the beta-crystals must be given sufficient time to form within the precursor. The amount of time depends upon numerous factors including, but not limited to, amount and type of nucleator, type of polypropylene, residence time/temperature; and type of equipment. Beta-crystal growth begins at a higher temperature than the alpha-crystal growth. Ideally, one should maximize the time of the polymer at a temperature above the initiation temperature of alpha-crystal growth but below the initiation temperature of beta-crystal growth. Optionally, the precursor may be annealed before stretching.
Stretching may be uniaxial or biaxial, but biaxial is preferred. Biaxial stretching includes a machine direction (MD) stretch, a transverse direction (TD) stretch, and optionally a relax (or stress relief) step. The MD stretch conditions include: temperature preferably ranging from 70-110xc2x0 C., most preferred at 90xc2x0 C.; and stretch ratio preferably ranging from 1.5-6.0, preferably 4. The TD stretch conditions include: temperature preferably ranging from 110-140xc2x0 C., most preferred at 120xc2x0 C.; and stretch ratio ranging from 1.5-6.0, most preferred at 4-5. During stretching, it is assumed that the polymer is at or near the stated temperatures.
Further details of the process are set forth in the examples below.